1. Field of the Invention
The present invention generally relates to a method and apparatus for preamble-less demodulation, and more particularly relates to a carrier recovery system and method and a clock recovery system and method for demodulating a signal (into which a unique word has been inserted), thereby demodulating a signal subjected to phase-shift keying (PSK) or frequency-shift keying (FSK).
2. Description of the Related Art
A conventional technique for demodulating a received signal (into which a unique word has been inserted) includes synchronizing a frame and sensing the unique word to demodulate the signal. The unique word is a fixed pattern which is known by the receiver side, and it is used typically for frame synchronization. For example, "a unique word sensor" described in Japanese Patent Application Laid-Open (Kokai) No. 5-167630 provides a circuit for sensing a unique word by a relatively small circuit, even if the input signal has a frequency error.
As shown in FIG. 9, a signal having a frame into which a unique word (e.g., a fixed word) is periodically inserted is referred to as an input signal R(t). This input signal R(t) is first delay-detected (e.g., differential-detected). Differential-detection is a known modulation scheme. In this case, differential-detection is utilized to find the unique word in the input signal. The differential-detection is conducted by a first delay-detector (e.g., differential-detector) 25, thereby obtaining a detected output D(t). Differential-detection is conducted using the equation of: EQU D(t)=R(t)R*(t-.tau.)
Next, a unique word pattern U(t) from a unique word generator 21 is differential-detected by a second differential-detector 26, thereby obtaining a pattern W(t). The cross-correlation between the detected output D(t) and the pattern W(t) is calculated by a cross-correlator 22. Briefly, such a cross-correlator 22 operates by comparing the pattern with known patterns, and utilizing, for example, the equation as follows: EQU C(t)=.intg.W*(.tau.)D(t+.tau.)d.tau.
The power converter 24 takes the square of the absolute value and uses the following equation: EQU P(t)=.vertline.C(t).vertline..sup.2
Electric power converter 24 issues a signal representing power (e.g., watts) to a level sensor 23. Level sensor 23 detects whether the absolute value of the calculated cross-correlation exceeds a threshold value, thereby sensing the unique word. The input signal is differential-detected by the above-mentioned structure, which is capable of removing a frequency offset included in the input signal. The frequency offset is generated by the frequency difference between a transmitter frequency fT(e.g., the transmitter side) and a receive frequency fR(e.g., the receiver side). If the frequency offset is not removed, the correlator c(t) cannot detect the unique word since correlation relies on the equation of .DELTA.f and if .DELTA.f is large, the patterns cannot be correlated, and the unique word cannot be found. Such an operation relies on the following equations: The relation between an orthogonal modulation data (I, Q) and an orthogonal demodulation data (I', Q') is ##EQU1##
Further, FIG. 10 illustrates a conventional technique for a carrier recovery circuit. A signal Rs is output from a fixed word table 32 through a switch unit 34. The fixed word (e.g., a unique word) portion in the input signal (Ri) is inverse-modulated with the signal Rs by inverse-modulator 36 (e.g., inverse-modulators 36 and 38 may be complex multipliers or the like for removing the modulation component from the received signal) and is complex-conjugated (e.g., converting the input complex signal to its conjugate value (a+jb.fwdarw.a-jb) by a multiplier (e.g., within de-modulator 36). An output signal of the inverse-modulation w(t) is determined by the following equation: ##EQU2##
.DELTA.f is the frequency offset. The inverse-modulator 36 removes a modulated component (Rd) from the signal Rs, and outputs the modulated component Rd to an adaptive line enhancer 31.
The adaptive line enhancer 31 is tuned to a line component of the above inverse-modulated signal Rd, and extracts a carrier wave component, thereby obtaining a demodulated output Ro of the input signal. For purposes of this application, a line component is expressed as: EQU e.sup.j2.pi..DELTA.f.tau.
With such a conventional structure, when data appears after the unique word portion of the input signal, the changing switch 34 is switched to a hard decision circuit 33. The data portion of the signal is detected since the relationship between the unique word and the data timing is always the same (e.g., constant). Thus, the data timing can be estimated from the unique word position. The hard decision circuit 33 estimates an integer Rh (fixed word) based on the demodulated signal Ro. This estimation is made by examining which quadrant the demodulation signal is (e.g., -1+j; 1+j; 1-j; or -1-j, where j is an imaginary value)).
The input signal Ri is inverse-modulated by the estimated value Rh (e.g., with element 36), thereby removing the modulated component from the input signal Ri. The details of the above-mentioned operation are described in Japanese Patent Application Laid Open (Kokai) No. 5-63743.
Therefore, in a conventional demodulating device, the position of the unique word in the input signal is sensed according to the above-mentioned technique of Japanese Patent Application Laid Open (Kokai) No. 5-167630 (e.g., FIG. 9). Then, based on the technique of Japanese Patent Application Laid Open (Kokai) No. 5-63743, the unique word portion is inverse-modulated according to a unique word table. The other portion (e.g., the data portion) is inverse-modulated by an output of the demodulator, thereby executing the modulating process.
In other words, the changing switch 34 shown in FIG. 10 is driven by an output of the level sensor 23 shown in FIG. 9, thereby selecting whether the inverse-modulation should be based on the unique word table 32 (e.g., R.sub.u) or the modulated output signal Ro (and R.sub.H from hard decision circuit 33).
According to the technique of Japanese Patent Application Laid Open (Kokai) No. 5-167630, the unique word detection pulse is output when the output of the correlator 22 exceeds the threshold value (e.g., when the clock phase of the input signal cannot be estimated with a high accuracy). In such a situation, the subsequent characteristic (such as timing) of the demodulator deteriorates. More specifically, the accuracy with which the clock phase is estimated (depending on how the threshold value is determined), falls within a range of about .+-.1/2 clock (e.g., one-half of a given clock cycle). This deviation is large enough to impact significantly the performance of a given device.
The structure illustrated in FIG. 10 (e.g., Japanese Patent Application Laid Open (Kokai) No. 5-63743) has an "acquisition" or "pull in" frequency range (in which the acquisition is realized at a high probability of, for example, over 95%) is only about .+-.1/8 of a symbol frequency. The "acquisition frequency" (also known as the "pull-in frequency" or "lock-in frequency") is related to the symbol frequency based on the acquisition probabilities along a graph (e.g., a Y-axis) and the frequency error offset along the X-axis, where the frequent error offset is from 0 to fs/4 where fs is equal to the symbol frequency.
A problem arises with the structure shown in FIG. 10 because an input signal which has an input frequency offset that exceeds the above range (e.g., 1/8) cannot be demodulated.